1. Field of the Invention
The present invention relates to a recording/reproducing apparatus with an optical pickup actuator capable of improving a gain margin and decreasing vibration by increasing a secondary resonant frequency, such that the optical pickup actuator can be stably operated at high speed.
2. Description of the Related Art
In general, a recording/reproducing apparatus can record/reproduce information on/from a recording medium (e.g., an optical disc), and may include a pickup actuator, e.g., an optical pickup actuator moved radially in and out over the disc to radiate light onto a recording surface of the disc and receive light reflected from the recording surface.
Referring to FIGS. 1 and 2, a conventional optical pickup actuator includes a holder 13 provided on one side of a base 10, and a bobbin 17 mounted on a center of the base 10, having an object lens 15. The bobbin 17 is formed at both ends thereof with through-holes 18 extending to the object lens 15. A first magnet 20 is inserted into the respective through-holes 18, and second magnets 23 are installed at both sides of the bobbin 17. The first and second magnets 20 and 23 are respectively attached to first and second yokes 25 and 27 provided on the base 10. Also, third magnets 30 are installed along both sides of the base in a track direction T of the base. The third magnets 30 are installed in third yokes 33 provided on the base 10.
One end of the bobbin 17 is suspended and movably supported by a suspension wire 35 fixed to the holder 13.
Meanwhile, the pickup actuator includes a magnetic driving unit for driving the bobbin 17 in a focusing direction F, a tracking direction T and a tilting direction t. The magnetic driving unit has a track coil 40 wound around an inner wall of the through-hole 18 of the bobbin 17, a focus coil 43 wound around an outer periphery of the through-hole 18, at bobbin portion 17b, and tilt coils 45 wound around both sides of the bobbin 17, at bobbin portion 17a, in the tracking direction T, in addition to the respective first through third magnets 20, 23 and 30.
When the focus coil 43, the track coil 40 and the tilt coil 45 are supplied with a power, the bobbin 17 is actuated in the focusing direction F, the tracking direction T or the tilting direction t by interacting with the respective first to third magnets 20, 23 and 30 to implement focusing, tracking and tilting operation of the object lens 15.
Since the bobbin 17 is suspended by the suspension wire 35, the bobbin has vibration characteristics. Accordingly, the pickup actuator includes an open loop for a gain and a phase, in accordance with a frequency, so as to measure the vibration characteristics. Referring to FIG. 3A, depicting a common open loop, a frequency “a” corresponding to a 0 dB gain results in a cutoff frequency for 0 dB, with a point “P” representing a secondary resonant peak. A gain difference between 0 dB and the secondary resonant peak represents a gain margin GM.
As recording media recording/reproducing speeds have steadily increased, the operating frequencies have increased in focusing and tracking modes of optical pickups, in accordance with the speed, causing deflection and eccentricity of the recording media. Therefore, it has become necessary to overcome such problems raised by disturbances of an eccentric disc and a deflection disc and increased acceleration, so as to permit fast-speed optical recording media operations. In order to solve the above problems, the cutoff frequency for 0 dB has to be increased. To this end, the gain and phase are changed in an RF chip, a drive IC chip and a digital equalizer (EDQ). However, a secondary resonance is inevitably generated at a frequency of above 20 kHz, because of a physical structure of the optical pickup actuator, and a gain is remarkably increased in a range of the secondary resonant frequency. The gain in a secondary resonant peak may exceed 0 dB, which means the gain margin is 0.
On that occasion where the gain margin is decreased, if the secondary resonant frequency, and a certain disturbance having the secondary resonant frequency divided by n (wherein, n is 1, 2, 3, . . . ), are input into the optical recording device, there is high possibility that a disturbance will be produced in the optical recording device. Thus, the gain margin at the secondary resonant peak becomes one of the most important factors in designing a controller at the fast-speed drive of the optical recording medium, because of the possibility of such a disturbance.
A method of ensuring the gain margin includes one of converting the secondary resonant frequency into a high frequency and the other of reducing a size of the secondary resonant peak. FIG. 3B depicts the method of ensuring the gain margin by converting the secondary resonant frequency into a high frequency. The gain margin is increased from GM1 to GM2 (GM1<GM2) by increasing the secondary resonant frequency from c to d.
Meanwhile, the bobbin is therefore generally made of liquid crystalline polymer (LCP) having high stiffness to increase the secondary resonance frequency. In this case, the secondary resonant frequency can be increased because of the high stiffness. However, since the absorbing vibration capability is poor, due to a low logarithmic decrement or low internal lost value, many high-frequency vibrating modes are produced, thereby decreasing the gain margin. Therefore, a composite material is generally used, where the LCP material is mixed with flake or fiber-type reinforcement such as carbon.
Although the bobbin is made of a material having high stiffness, to suppress the high-frequency vibration mode, as well as to increase the secondary resonant frequency and thereby improving the gain margin, there are limited selections in available materials.